The preparation of vinyl acetate by reacting ethylene with acetic acid and oxygen or oxygen-containing gases in the gas phase over a fixed-bed catalyst has been known for a long time. The starting materials are reacted in an exothermic reaction, generally at a pressure of from 1 to 30 bar and at a temperature of from 130° C. to 200° C., in a fixed-bed tube reactor or fluidized-bed reactor to form vinyl acetate according to the following overall equation:C2H4+CH3COOH+½O2→CH3COOCH═CH2+H2O
The product gas stream leaving the reactor contains vinyl acetate together with essentially unreacted starting materials, water and also inerts and by-products such as carbon dioxide, acetaldehyde, methyl acetate and ethyl acetate. Inerts are essentially nitrogen, argon, methane and ethane and are introduced into the process as impurities in the starting materials.
To isolate pure vinyl acetate from the product gas stream, many process variants have been proposed. In DE-A 1282014 and DE-A 1668063, it is recommended that the product gas stream be introduced into a first distillation column from which water and low boilers are distilled off at the top and the bottoms are transferred to a second distillation column from which pure vinyl acetate is distilled off at the top. To separate methyl acetate and ethyl acetate from mixtures with vinyl acetate, DE-A 1618240 recommends water-operated extraction processes. U.S. Pat. No. 3,692,636, U.S. Pat. No. 4,934,519 and U.S. Pat. No. 6,228,226 describe azeotropic distillations of mixtures containing vinyl acetate, water, ethyl acetate and acetic acid.
In the process of DE-A 1768412, condensable constituents such as vinyl acetate, acetic acid and water are condensed out from the product gas stream and are introduced as condensates into a first distillation column (azeotropic column) from which vinyl acetate and water are distilled off at the top and are subsequently subjected to a phase separation. The phase containing vinyl acetate is introduced into a second distillation column (dewatering column), the bottoms from which are transferred to a third distillation column (pure vinyl acetate column) from which pure vinyl acetate is obtained at the top. In addition to the abovementioned process steps, the gas phase remaining after condensation of the product gas stream is scrubbed with acetic acid (recycle gas scrubber) in the processes of DE-A 2945913 and DE-A 2943985; the scrubbing solution obtained is introduced into the azeotropic column.
To achieve energy-saving removal of water, DE-A 2610624 recommends introducing the product gas stream into a preliminary dewatering column from which water and vinyl acetate are distilled off at the top and are separated after condensation and phase separation. The phase containing vinyl acetate obtained in this way is recirculated to the preliminary dewatering column or introduced together with the bottoms from the preliminary dewatering column into a dewatering column, the bottoms from which are subjected to further distillative purification steps to obtain pure vinyl acetate. In the processes of EP-A 1760065, DE-A 102006038689, DE 3934614, DE 3422575 and U.S. Pat. No. 4,818,347, the above-described preliminary dewatering column is combined with a recycle gas scrubber operated using acetic acid, an azeotropic column and a dewatering column and also a pure vinyl acetate column, with the pure vinyl acetate being obtained from the top of the latter column.